compute: bucket temporally on the columnar edge

[antiguru]

Makes temporal bucketing columnar throughout, removing the four Vec round-trips its call sites paid. The operator's state was already columnar, since its bucket chain wraps ColumnMergeBatcher, so the decode and re-encode existed only at its boundary.

Three per-record costs go with the round-trips. The reveal path handed the output builder owned tuples reconstituted from the chain's Column chunks, and now gives each chunk as a container. MergeBatcherWrapper::push_container staged its Vec argument into a Column one record at a time before the chunker could read it, and now takes a Column. Bucket::split round-tripped the sealed chunks through a Vec, cloning every record, and now re-chunks the containers.

On the input side, records are addressed by index through a permutation rather than moved, so neither the pass-through nor the bucket routing materializes an owned record. The partition happens before the sort, because only retained records need ordering and in steady state the pass-through share is the larger one.

TemporalBucketing::bucket now returns Self, which lets a Vec stream and a columnar stream both implement it. The reduce key-value path stays on the Vec implementation, which stages through the columnar operator so there is one bucketing implementation rather than two. It feeds an arrangement, and a consumer that re-encodes what it reads is cheaper to hand moved allocations than copied bytes.

Worth knowing for review: enable_compute_temporal_bucketing is compiled in as false but served on in production, so this is a live path rather than a dormant one. CPU-213 tracks reconciling the default. Verified with the flag defaulted on, since the compiled-in default otherwise leaves the operator untested outside temporal_bucketing.slt.

_{Posted by Claude Code}

[linear-code]

CPU-51